1. Field of the Invention
The present inventions relates to atomic stabilized frequency sources and more particularly to an improved adjustable frequency synthesizer adapted for use with such sources.
2. Prior Art
Although the present invention may find practical application in any one of the numerous atomic stabilized frequency sources, it is particularly adaptable for operation in a rubidium vapor cell frequency standard. Rubidium vapor cell frequency standards, as well as other types of atomic stabilized frequency sources, are described extensively in the literature. For example, reference may be had to the texts respectively entitled, "Frequency and Time" by P. Kartaschoff, Academic Press, 1978, and "Frequency Synthesizers Theory and Design", Second Edition, by Vadim Manassewitsch, John Wiley and Sons, 1980. Such frequency sources are stabilized by quantum mechanical atomic state transition resonances such as the hyperfine atomic resonant frequency related to the change in the internal energy of the atom. A rubidium frequency standard operates as a discriminator based upon the energy absorption characteristic of rubidium-87. In practice a rubidium lamp passes a light beam into a rubidium absorption cell. The ribidium cell absorbs some of the light energy because of the energy level transitions in the rubidium-87 gas. When an electromagnetic field of frequency equal to the resonant frequency of the rubidium vapor is applied to the vapor cell, the number of energy level transitions in the rubidium-87 gas is increased and more of the light emitted by the rubidium lamp is absorbed in the rubidium vapor cell. Typically, a photodiode is used to detect the occurrence of the maximum absorption of light from the rubidium lamp which occurs when the frequency of the exictation electromagnetic field exactly matches the rubidium resonant frequency. Typically, a frequency synthesizer is used to generate the appropriate electromagnetic field frequency of approximately 6,834.685 MHz. This field is frequency modulated at a relatively slow rate (i.e., 154 Hz.) so that the photodiode provides a demodulated signal which may be applied to a phase detector or comparator which also receives the reference modulation signal. The output of the phase comparator is a DC error voltage which is used to control a voltage controlled crystal oscillator at a selected frequency, typically of 5 or 10 MHz. In this manner, the frequency of the crystal oscillator is stabilized approximately to one part per 10.sup.11 or better over long periods of time to provide a highly stable and accurate frequency source.
Usually, because of variations in the precise gas pressure within the rubidium vapor cell, or variations in the quantity of rubidium gas relative to carrier gases that are mixed with the rubidium gas to achieve desired temperature insensitivity, the hyperfine transition resonance frequency of the optically pumped absorption cell varies to some extent from cell to cell. As a consequence, in those prior art rubidium frequency standards in which a fixed frequency synthesizer is used to develop the electromagnetic excitation field applied to the cell, it is necessary to vary the resonance frequency of the cell by altering the pressure of the gases in the cell. Although some resonance frequency variation may be achieved by changing the intensity of a magnetic field applied to the cell, the extent to which variation may be achieved in this manner is usually very limited and it is generally impractical to provide sufficient adjustment of the resonance frequency to accommodate the fixed frequency of the electromagnetic field using only the magnetic field. As a consequence thereof, some manufacturers of rubidium frequency standards utilize an iterative process of finely adjusting the gas pressure by dimpling or otherwise changing the geometry of the gas cell structure, each time rechecking the resonance frequency of the cell. Unfortunately, this iterative process is time consuming and expensive and often results in a substantial reduction of cell production yield because of the irreversible nature of the dimpling process and the high risk of physical damage to the cell structure.
Although there are prior art references which disclose rubidium frequency standards employing frequency synthesizers of which the output frequency may be varied over some limited range, the adjustability of such synthesizers is primarily intended for an alternative purpose and therefore does not provide the requisite degree of variation needed to assure 100% cell yield due to moderate variations in gas pressure and mixture content as noted above. For example, there are those applications in which the frequency stable signal provided by the frequency source serves as a measure of time relative to the current ephemeris time scale. In such applications it is desireable to periodically change the ephemeris time scale which does not remain fixed relative to earth rotation. Consequently, it is necessary to adjust the output frequency of the atomic clock frequency standard in order to maintain an accurate measure of time. One way of adjusting the output frequency of the atomic clock frequency standard, without the replacement or modification of resonator parts, is to provide a variable frequency synthesizer which permits maintaining the hyperfine transition resonance center frequency of the atomic frequency standard by instead altering the frequency of the crystal controlled oscillator relative to the resonant center frequency of the standard. However, the frequency offset necessary to accommodate time scale changes for earth time correction need only be on the order of a fraction of a Hz. As a result, such prior art adjustable frequency synthesizers used with rubidium frequency standards tend to have extremely limited ranges which cannot accommodate larger deviations in cell resonance frequency. Furthermore, such prior art adjustable frequency synthesizers tend to be very complex and therefore unreliable and often require a plurality of adjustments to achieve the desired offset frequency. Often such prior art standards still require some adjustment of the magnetic field because of inadequate resolution of the frequency synthesizer increments or because of the complexity of the synthesizer adjustment which renders adjustment of the magnetic field more desirable.
The most relevant prior art known to the applicant is disclosed in U.S. Pat. No. 3,363,193 to Arnold; U.S. Pat. No. 3,408,591 to Helgesson; and in the Hewlett Packard manual entitled "Rubidium Vapor Frequency Standard 5065A" published by the Hewlett Packard Company in February 1970. The Arnold patent discloses an adjustable synthesizer with an offset frequency adjustment range of only + or -204 Hz. and which requires a plurality of manual adjustments including use of a four-pole, seven-position switch. The patent to Helgesson, although disclosing a synthesizer of somewhat broader range, (i.e. + or -1400 Hz.) utilizes a very complex phase-to-voltage converter locking circuit which is inherently unreliable. As a result thereof, Helgesson's synthesizer requires special alarm circuits to indicate an unlocked or false locked condition which would otherwise adversely affect the accuracy of the offset frequency setting. In addition, in order to achieve even the limited range disclosed in that patent it is apparently necessary to change crystals in the voltage controlled crystal oscillator; a highly undesirable inconvenience.
Frequency offset in the Hewlett Packard apparatus is accomplished by a plurality of manual adjustments including the setting of four thumbwheel switches and that apparatus still only provides a total offset range of about 684 Hz.. Furthermore the manufacture specifies a preference for controlling frequency offset by varying the magnetic field, resorting to electronically changing the offset frequency only when the magnetic field variation is inadequate to accomplish the desired offset.
Furthermore, it is to be noted that prior art frequency synthesizers used in rubidium frequency standards serve the sole purpose of controlling the precise frequency of electromagnetic field applied to the rubidium vapor cell. Accordingly, a separate oscillator must be used to phase modulate the output signal of the voltage controlled crystal oscillator to provide the requisite error signal detected by the photodiode which is then used to develop the DC signal to control the crystal oscillator within the feedback loop previously noted. The use of a separate modulation oscillator introduces some risk of error in detecting the precise rubidium cell resonance frequency. Furthermore, phase modulation makes it difficult to measure the resultant modulation at the very high frequency of the applied electromagnetic field because it is difficult to see the deviation at lower frequencies closer to the unmultiplied output of the voltage controlled crystal oscillator.